In recent years, the development of a magneto-optical memory element having a high capacity and storage density, and capable of recording and/or erasing repeatedly, has been actively pursued. This type of magneto-optical memory element is usually composed by a magnetic thin film formed on a substrate. The magnetic thin film is provided with an axis of easy magnetization perpendicular to the surface of the film, and is magnetized in either direction through initialization.
During recording, a relatively strong laser beam is irradiated on the magnetic thin film, while applying an external magnetic field of a direction opposite to the direction of the external magnetic field which was applied during the initialization. In the part irradiated, the temperature rises causing the coercive force to lower. As a result, the magnetization is inverted in the direction of the external magnetic field.
During reproduction, a relatively weak laser beam is irradiated on the magnetic thin film. The plane of polarization of the light reflected from the magnetic thin film is rotated in accordance with the direction of magnetization through the magneto-optical effect. Information signals can be thus detected by detecting the inclination of the plane of polarization.
An example of a conventional optical pickup device adapted for a magneto-optical memory element is illustrated in FIG. 26.
A linearly polarized laser beam projected from a semiconductor laser 51, is converted into a parallel beam of light by a collimating lens 52, passes through a compound beam splitter 53 and is converged on a magneto-optical memory element 56 across a mirror 54 and an objective lens 55.
The plane of polarization of the reflected light from the magneto-optical memory element 56 is rotated in accordance with the direction of magnetization at the time of the reflection. The reflected light is then led across the objective lens 55 and the mirror 54 to the compound beam splitter 53 where it is reflected in direction at a right angles to the incidence direction of the reflected light by a face 53a. A portion of the reflected light is further reflected in a direction at right angles to the incidence direction of the reflected light by a face 53b, passes through a spot lens 57 and a cylindrical lens 58, and impinges upon a photodetector 60 of a four-quadrant type. In the photodetector 60, a tracking error signal and a focus error signal are generated based on the widely known push-pull method and astigmatic method respectively.
Meanwhile, the light that was transmitted through the face 53b of the compound beam splitter 53 is reflected in a direction at a right angle with the incidence direction of the transmitted light by a face 53c, and its plane of polarization is rotated by 45.degree. by a half-wave plate 59. This light is then split into two polarized lights having mutually orthogonal polarizations by a polarizing beam splitter 61, and the two polarized lights impinge on the photodetector 62 and the photodetector 63 respectively. Information on the magneto-optical memory element 56 is reproduced based on the output signals of the photodetectors 62 and 63.
In the magneto-optical memory element 56, the detection of information signals is generally performed by making use of the Kerr effect.
In FIG. 27, suppose that the laser beam irradiated on the magneto-optical memory element 56 is, as shown by I, a linearly polarized light of a P polarization only. When the direction of magnetization in the part of the magneto-optical memory element 56 that was illuminated by the laser beam, coincides with the upward direction in FIG. 26, the plane of polarization of the reflected light is rotated by (+.epsilon..sub.k), as shown by II in FIG. 27. On the other hand, when the direction of magnetization in the part that was illuminated by the laser beam, coincides with the downward direction in FIG. 26, the plane of polarization of the reflected light is rotated by (-.epsilon..sub.k) as shown by III in FIG. 27. Accordingly, information on the magneto-optical memory element 56 can be reproduced by detecting the rotation of the plane of polarization by means of the photodetectors 62 and 63.
However, the Kerr angle of rotation .epsilon..sub.k is generally an extremely small angle of 0.5.degree. to 1.5.degree.. The Kerr angle of rotation .epsilon..sub.k needs thus to be given a bigger more easily detectable appearance in order to obtain a reproduction signal of a high quality.
Hence, in the optical pickup device shown in FIG. 26, the angle .epsilon..sub.k is made bigger in appearance by giving a polarization property to the face 53a or 53b of the compound beam splitter 53.
For example, the face 53b may be designed such that the transmittance T.sub.P for the P polarization is set at 30% and the reflectance R.sub.P is set at 70%, and the transmittance T.sub.S for the S polarization is set at 100% and the reflectance R.sub.S is set at 0%. As a result, as illustrated in FIG. 28, the P polarization that passed through the face 53b is reduced to 30%, while the S polarization is not reduced. The apparent Kerr angle of rotation .epsilon..sub.k ' is thus increased and equals approximately 1.0.degree. to 2.7.degree..
However, the use of the compound beam splitter 53, the polarizing beam splitter 61 and other members in the optical pickup device shown in FIG. 26, causes the number of parts as well as the weight of the device to increase, and as the weight of the device increases it correspondingly causes the access time to be slow.
Another example of an optical pickup device adapted for the magneto-optical memory element 56 is illustrated in FIG. 29. The composing members common to the optical pickup device of FIG. 26 and the optical pickup device of FIG. 29 are designated by the same reference numerals.
A laser beam is projected from a semiconductor laser 51, passes through a Kerr angle of rotation multiplier prism 64, is converted into a parallel beam of light by a collimating lens 52, and is converged on the magneto-optical memory element 56 across an objective lens 55.
The reflected light from the magneto-optical memory element 56 passes through the objective lens 55 and the collimating lens 52. A portion of the reflected light is further reflected in a direction at right angles to the incidence direction of the reflected light by the Kerr angle of rotation multiplier prism 64. The reflected light then passes successively through a half-wave plate 59, a cylindrical lens 58 and a concave lens 65, and impinges upon a polarizing beam splitter 66.
In the polarizing beam splitter 66, the reflected light from the magneto-optical memory element 56 is split into two polarized lights having mutually orthogonal polarizations. One of the polarized lights is transmitted through the polarizing beam splitter 66 and impinges on a photodetector 60 of a four-quadrant type. A focus error signal and a tracking error signal are then generated by following the same process as in the photodetector 60 of FIG. 26.
As to the other polarized light, it is reflected at right angles by the polarizing beam splitter 66 and impinges upon a photodetector 67. Information on the magneto-optical memory element 56 is reproduced by performing the operation of the signal that was released by the photodetector 67 and the signal produced by summing the signals released by the different photodetection sections of the photodetector 60, and amplifying the result of the operation.
In the optical pickup device of FIG. 29, the separation of the reflected light from the magneto-optical memory element 56 is performed by the polarizing beam splitter 66. However, for manufacturing reasons, it is difficult to fabricate the polarizing beam splitter 66 so that each side measures approximately less than 2 mm, thereby causing the polarizing beam splitter 66 to be large and heavy. A similar difficulty arises with the Kerr angle of rotation multiplier prism 64. In addition, the optical pickup device suffers from a drawback as its access speed when it moves to a desired radial position on the magneto-optical memory element 56, lowers because of the weight increase due to the Kerr angle of rotation multiplier prism 64 and the polarizing beam splitter 66.
Further, another example of optical pickup device adapted for the magneto-optical memory element 56 is illustrated in FIG. 30.
A laser beam is projected from a semiconductor laser 51, converged on the magneto-optical memory element 56 across a collimating lens 52, a first prism section 68a of a Kerr angle of rotation multiplier compound prism 68 and an objective lens 55. The reflected light from the magneto-optical memory element 56 is led to the first prism section 68a of the Kerr angle of rotation multiplier compound prism 68 across the objective lens 55. A portion of the reflected light is reflected in a direction at right angles to the incidence direction of the reflected light by the first prism section 68a and is further separated into a transmitted light and a reflected light by a second prism section 68b.
The light that was transmitted through the second prism section 68b impinges upon a Wollaston prism 70 across a half-wave plate 59 and a convex lens 69. The transmitted light is split by the Wollaston prism 70 into two polarized lights which impinge respectively upon the different photodetection sections of a photodetector 71 of a two-divisions type. Information on the magneto-optical memory element 56 is reproduced by performing the operation of the signals that were generated based on the two polarized lights detected on the different photodetection sections, and amplifying the result of the operation.
On the other hand, the reflected light that was reflected in a direction at right angles to the incidence direction of the reflected light, by the second prism section 68b of the Kerr angle of rotation multiplier compound prism 68, impinges upon a photodetector 60 of a four-quadrant type across a convex lens 72 and a cylindrical lens 58. A focus error signal and a tracking error signal are then generated based on the signals released by the photodetector 60.
However, with the optical pickup device of FIG. 30 also, it is difficult to form the Wollaston prism 70 so that each side measures approximately less than 2 mm. Further, as the Kerr angle of rotation multiplier compound prism 68 is of a large size also, this causes the whole optical pickup device to be large and heavy. Moreover, as the Wollaston prism 70 and other parts are made of a crystalline material, the cost of the optical pickup device is high.
A birefringent wedge 73 may be used instead of the Wollaston prism 70 in the optical pickup device of FIG. 30 (see FIG. 31). However, the deflection angle between the two polarizations P and S obtained with the birefringent wedge 73 equals for example 2.06.degree., which is smaller than the deflection angle equal to 4.6.degree. obtained with the Wollaston prism 70. The birefringent wedge 73 is thus more disadvantageous than the Wollaston prism 70 in terms of size of the device.
As described above, when, in order to split the reflected light from the magneto-optical memory element 56, using the polarizing beam splitters 61 and 66, the Wollaston prism 70 and the birefringent wedge 73, or mounting the compound beam splitter 53 or the Kerr angle of rotation multiplier prism 64 between the semiconductor laser 51 and the magneto-optical memory element 56, the consequence of a large and heavy optical pickup device cannot be avoided.
Hence, recent experiments have used a diffraction element having a desired polarization property as a means for cutting down the number of parts and reducing the weight of the optical pickup device. A diffraction element having the desired polarization property will be discussed hereinafter.
In the conventional art, a diffraction grating which grating pitch is formed approximately equal to a predesignated wavelength of the light is known to have a particular polarization property (K. Yokomori, "Dielectric surface-relief gratings with high diffraction efficiency," Applied Optics Vol. 23, No. 14, pp 2303, 1984).
As illustrated in FIG. 32, a polarization diffraction element 81 is composed of a diffraction grating 83 formed on one side of a transparent substrate 82 made of glass or other material according to the two beam interference method or other method. The diffraction grating 83 has a polarization property and its grating pitch is formed approximately equal the wavelength of the light it is designed for. The diffraction grating 83 is made of, for example, photoresist and its thickness and grating pitch are respectively set at 1 .mu.m and 0.5 .mu.m. The diffraction grating 83 is fabricated so that a P polarization is transmitted at virtually 100%, and a S polarization is diffracted at virtually 100%.
When an incident light 84 with a wavelength of for example 0.8 .mu.m impinges upon the polarization diffraction element 81 at a Bragg angle, the P polarization of the incident light 84 is transmitted through the diffraction grating 83 to produce a zeroth-order diffracted light 84a while it is virtually not diffracted and produce virtually no first-order diffracted light 84b. On the other hand, the S polarization of the incident light 84 is diffracted by the diffraction grating 83 to produce a first-order diffracted light 84b but is virtually not transmitted and produces virtually no zeroth-order diffracted light 84a.
In order to detect the zeroth-order diffracted light 84a and the first-order diffracted light 84b after they are separated as described above, the zeroth-order diffracted light 84a is converged on a photodetector 87 across a converging lens 85 and the first-order diffracted light 84b is converged on a photodetector 88 across a converging lens 86.
As described above, the diffraction grating 83 has the property of splitting light of different polarizations. The polarization diffraction element 81 that comprises the diffraction grating 83 may be thus employed as a polarizing beam splitter in an optical pickup device used in a magneto-optical recording and reproducing apparatus. By using the polarization diffraction element 81 as a polarizing beam splitter, the number of parts may be reduced and the optical pickup device can be made compact and light.
An example of an optical pickup device provided with a polarization diffraction element such as described above is illustrated in FIG. 33. The composing members common to the optical pickup device of FIG. 26 and the optical pickup device of FIG. 33, are designated by the same reference numerals.
In FIG. 33, a linearly polarized laser beam that was projected from a semiconductor laser 51 is irradiated on the magneto-optical memory element 56 across a collimating lens 52, a beam splitter 74, a mirror 54 and an objective lens 55.
The reflected light whose plane of polarization was rotated on the magneto-optical memory element 56 in accordance with a recording signal, reaches the beam splitter 74 across the objective lens 55 and the mirror 54. The reflected light is reflected in a direction at a right angle with the incidence direction of the reflected light by the beam splitter 74, and impinges upon a polarization diffraction element 77 across a half-wave plate 75 and a converging lens 76 thereafter.
The polarization diffraction element 77, as illustrated in FIG. 34, is for example divided into four areas in order to generate a servo signal. Diffraction gratings 77a, 77b, 77c and 77d are respectively mounted on each of the areas. The grating pitch of each of the diffraction gratings 77a, 77b, 77c and 77d, is approximately equal to the wavelength of the laser light it is designed for.
A zeroth-order diffracted light that was transmitted through the polarization diffraction element 77 is split into two polarized lights by a birefringent wedge 78. Information on the magneto-optical memory element 56 is reproduced as the two polarized lights impinge on the different photodetection sections of a photodetector 79 of a two-divisions type.
Meanwhile, a first-order diffracted light that was diffracted by the polarization diffraction element 77 impinges upon a photodetector 80 of a multi-divisions type. A tracking error signal and a focus error signal are then generated by performing the operation of the output signals released by the different photodetection sections of the photodetector 80.
In the polarization diffraction element 77, the Kerr angle of rotation may be made bigger in appearance for example by setting the zeroth-order diffraction efficiency at 30% and the first-order diffraction efficiency at 70% for the P polarization, and setting the zeroth-order diffraction efficiency at 100% and the first-order diffraction efficiency at 0% for the S polarization.
Moreover, an optical pickup device such as the one illustrated in FIG. 35, may be fabricated for example by employing diffraction gratings instead of the Kerr angle of rotation multiplier prism 64 and the polarized beam splitter 66 of FIG. 29.
In the optical pickup device of FIG. 35, a linearly polarized laser beam L.sub.1 is projected from a semiconductor laser 89. The light L.sub.1 is split by a diffraction grating 90a formed on a substrate 90b in a diffraction element 90, to produce a zeroth-order diffracted light L.sub.20 and a first-order diffracted light L.sub.21 that is diffracted at an angle of diffraction .beta..sub.1. The first-order diffracted light L.sub.21 is then irradiated on a magneto-optical memory element 91 provided with a recording film 91b made of a magnetic thin film, and with a substrate 91a.
The first-order diffracted light L.sub.21 is irradiated on the recording film 91b and is reflected after its plane of polarization is rotated through the magneto-optical effect. The reflected light L.sub.3 is split by the diffraction grating 90a to produce a zeroth-order diffracted light L.sub.40 that is transmitted light, and a first-order diffracted light L.sub.41 that returns toward the semiconductor laser 89. The plane of polarization of the zeroth-order diffracted light L.sub.40 is rotated by 45.degree. by a half-wave plate 92 and the zeroth-order diffracted light L.sub.40 impinges upon a diffraction grating 93a formed on a substrate 93b in a diffraction element 93 thereafter. The zeroth-order diffracted light L.sub.40 is split to produce a zeroth-order diffracted light L.sub.50 that is transmitted, and a first-order diffracted light L.sub.51 that is diffracted at an angle of diffraction .beta..sub.1. The zeroth-order diffracted light L.sub.50 and the first-order diffracted light L.sub.51 are respectively received by the photodetectors 94 and 95. Information on the magneto-optical memory element 91 is reproduced by amplifying the output signals of the photodetectors 94 and 95 in a differential amplifier 96. When necessary, convex lenses 97 and 98 may be placed between the diffraction grating 93 and the photodetectors 94 and 95, as illustrated in FIG. 36.
FIG. 37 shows a graph illustrating the diffraction efficiency of the two mutually orthogonal polarizations for the zeroth-order diffracted light L.sub.40 and the first-order diffracted light L.sub.41 produced by the diffraction grating 90a, as a function of the groove depth of the grating. In FIG. 37, it is supposed that in the optical pickup device, the grating pitch of the diffraction grating 90a equals 0.59.alpha. (where .alpha. is the wavelength of the relative laser light), and the index of refraction of the substrate 90b equals 1.45. Here, L.sub.40 (TE) represents the polarization in the zeroth-order diffracted light L.sub.40, whose polarization is parallel with the grating lines of the diffraction grating 90a (the direction perpendicular to the paper surface in FIG. 35), and L.sub.41 (TE) represents the polarization in the first-order diffracted light L.sub.41, whose direction is parallel with the grating lines of the diffraction grating 90a. Similarly, L.sub.40 (TM) represents the polarization in the zeroth-order diffracted light L.sub.40 which direction is perpendicular to the grating lines of the diffraction grating 90a, and L.sub.41 (TM) represents the polarization in the first-order diffracted light L.sub.41 whose direction is perpendicular to the grating lines of the diffraction grating 90a.
As it is clearly shown in the figure, the ratio of the diffraction efficiency of L.sub.40 (TM):the diffraction efficiency of L.sub.41 (TM) almost equals 100:0 while the ratio of the diffraction efficiency of L.sub.40 (TE):the diffraction efficiency of L.sub.41 (TE) varies in accordance with the groove depth of the diffraction grating 90a.
In the conventional pickup device illustrated in FIG. 29, the P polarization (corresponding to the above TE polarization) of the reflected light from the magneto-optical memory element 56 is transmitted through and reflected on the Kerr angle of rotation multiplier prism 64 with a ratio set to about 70:30. Meanwhile the S polarization (corresponding to the above TM polarization) of the reflected light from the magneto-optical memory element 56 is transmitted through and reflected on the Kerr angle of rotation multiplier prism 64 with a ratio set to almost 0:100. Accordingly, characteristics almost equivalent the characteristics of the conventional Kerr angle of rotation multiplier prism 64 can be created in the diffraction grating 90a of FIG. 35 by setting L.sub.40 (TE):L.sub.41 (TE).apprxeq.30:70, and L.sub.40 (TM):L.sub.41 (TM).apprxeq.100:0. As it is clearly shown in FIG. 37, the groove depth satisfying these conditions is approximately equal to 0.77.alpha.. While with the Kerr angle of rotation multiplier prism 64 of FIG. 29, information signals are detected through the reflected light, with the diffraction grating 90a of FIG. 35, information signals are detected through the zeroth-order diffracted light L.sub.40 (transmitted light). The diffraction grating 90a is thus designed such that the TM polarization of the reflected light from the magneto-optical memory element 91 is transmitted at virtually 100%.
In addition, the diffraction element 93 is for example composed of a diffraction grating 93a of the same pitch and of the same direction as the diffraction grating 90a, formed on a substrate 93b of the same index of refraction as the substrate 90b. In order to give a polarization property to the diffraction grating 93a, its groove depth is set at approximately 1.2.alpha.. As a result, for the TE polarization that is parallel to the grating lines, L.sub.50 (TE):L.sub.51 (TE).apprxeq.0:100, while for the TM polarization that is perpendicular to the grating lines, L.sub.50 (TM):L.sub.51 (TM).apprxeq.100:0. Consequently, with the diffraction grating 93a, the zeroth-order diffracted light L.sub.50 is almost composed entirely of a TM polarization while the first-order diffracted light L.sub.51 is almost entirely composed of a TE polarization. Moreover, as the zeroth-order diffracted light L.sub.40 is rotated by 45.degree. by the half-plate 92 located short of the diffraction element 93, the axis serving as a reference for the separation of the polarized lights produced by the diffraction grating 93a, is rotated by 45.degree. with respect to the linearly polarized light projected from the semiconductor laser 89.
However, in the conventional polarization diffraction element 81 shown in FIG. 32, the diffraction angle of the light diffracted by the diffraction grating 83 is dependant on the wavelength of the incident light 84. For example when a laser diode is used as a light source, the wavelength of the light projected from the laser diode changes in accordance with the variation in the ambient temperature thereby causing the aforementioned diffraction angle to vary.
For instance, when the wavelength of the incident light 84 equals a predetermined wavelength, the first-order diffracted light 84b is diffracted at a predetermined diffraction angle and is converged accurately on the photodetector 88 by the converging lens 86. At this time, if the ambient temperature lowers and the wavelength of the incident light 84 becomes shorter than the predetermined wavelength, the diffraction angle will consequently become smaller causing the first-order diffracted light 84b that was diffracted by the diffraction grating 83 to deviate greatly off the predetermined optical path as shown by the chain double-dashed line in the figure. This gives rise to the inconvenience that the first-order diffracted light 84b cannot be converged on the prescribed position on the photodetector 88 and that the detection of the S polarization cannot be performed.
As described above, in the polarization diffraction element 81, the grating pitch of the diffraction grating 83 is set so as to be approximately equal to the wavelength. As a result, a slight change in the wavelength of the laser light of the incident light 84 causes the diffraction angle to vary greatly and causes the optical path of the first-order diffracted light 84b to deviate. The first-order diffracted light 84b that deviates off the optical path can be received by making the light receiving portion of the photodetector 88 bigger. However, this represents a disadvantageous factor when aiming at producing an optical pickup device that is compact and light, as the optical pickup device gets large when the polarization diffraction element 81 is incorporated therein together with the photodetector 88. Besides, even if the photodetector 88 is made bigger, difficulties arise as the focus of the first-order diffracted light 84b is not formed at a constant position on the photodetector 88 because of variations in the wavelength of the incident light 84 thereby causing a decline in the accuracy of the detection of the S polarization.
Further, as the diffraction angle of the first-order diffracted light 84b equals approximately 100.degree., the first-order diffracted light 84b and the zeroth-order diffracted light 84a travel at a great distance from each other. The photodetectors 87 and 88 have thus to be mounted in distant positions.
The same difficulties arise with the optical pickup device shown in FIG. 33 and FIG. 35. For instance in FIG. 35, the diffraction angle .beta..sub.1 of the diffraction grating 93a is quite big and equals approximately 100.degree. to 120.degree.. The photodetectors 94 and 95 thus need to formed independently and positioned in different directions. In addition, when a variation occurs in the wavelength of the laser beam projected from the semiconductor laser 89, the focal position of the first-order diffracted light L.sub.51 on the photodetector 95 shifts and in extreme cases slips off the photodetector 95 as shown in FIG. 36, whereby the detection of information signals cannot be performed. Consequently, like in the aforementioned case, it is difficult to produce an optical pickup device that is compact and light. Furthermore, setting the relative positions of the diffraction grating 93a and the photodetectors 94 and 95 is difficult.
The diffraction grating 83 of FIG. 32 is fabricated so that the P polarization is transmitted at virtually 100%, and the S polarization is diffracted at virtually 100%. Suppose a diffraction efficiency .delta..sub.0P represents the diffraction efficiency when the P polarization is transmitted to produce the zeroth-order diffracted light 84a, and a diffraction efficiency .delta..sub.1S represents the diffraction efficiency when the S polarization is diffracted to produce the first-order diffracted light 84b. In practice, when the incident light 84 impinges upon the single diffraction grating 83, the diffraction efficiency .delta..sub.0P and the diffraction efficiency .delta..sub.1S are both equal to approximately 0.99. Consequently, the zeroth-order diffracted light 84a that passed through the diffraction grating 83, contains a small amount of S polarization that was transmitted through the diffraction grating 83 with a diffraction efficiency .delta..sub.0S equal to approximately 0.01. Similarly, the first-order diffracted light 84b that was diffracted by the diffraction grating 83, contains a small amount of P polarization that was diffracted by the diffraction grating 83 with a diffraction efficiency .delta..sub.1P equal to approximately 0.01.
Accordingly, when splitting the incident light 84 by means of the single diffraction grating 83, the ratio of the diffraction efficiency .delta..sub.0S to the diffraction efficiency .delta..sub.0P shows the proportion of other polarizations contained in the desired polarization, i.e. the degree of polarization, for the zeroth-order diffracted light 84a. Similarly, the ratio of the diffraction efficiency .delta..sub.1P to the diffraction efficiency .delta..sub.1S shows the degree of polarization for the first-order diffracted light 84b. Consequently, when determining the degree of polarization of the zeroth-order diffracted light and of the first-order diffracted light, both are found to be equal to about 0.01. Therefore the degree of separation of the P polarization and S polarization is not of a level sufficient for practical use.
Moreover, in the optical pickup device shown in FIG. 33, a difference resulting from the polarization property of the diffraction grating 77a, 77b, 77c or 77d of FIG. 34, occurs between the phases of the P polarization and S polarization contained in the zeroth-order diffracted light produced by the polarization diffraction element 77. The zeroth-order diffracted light that was transmitted through the polarization diffraction element 77, thus become an elliptically polarized light thereby causing a decline in the quality of the reproduction signal.
As the above phase difference is due to the polarization property of the polarization diffraction element 77, it cannot be suppressed by optimizing the design of the polarization diffraction element 77. The phase difference can be compensated for instance, by inserting a phase compensating plate, not shown, between the polarization diffraction element 77 and the birefringent wedge 78. However this causes the number of parts to increase. The optical pickup device shown in FIG. 35 also presents a similar problem.
Phase shifting elements and antireflection elements are fundamental optical elements used in optical devices such as the optical pickup devices described above.
A phase shifting element controls the phase difference of two polarizations of mutually orthogonal polarizations, and in the conventional art is fabricated by using a crystalline body. FIG. 38 illustrates a conventional example.
A phase shifting element 99 is made of a crystalline plate of a thickness T, for example a quartz plate, and is mounted so that its optical axis is parallel to an axis X. For example, a system may include a quartz plate designed for a particular light having has a wavelength of 780 nm. The index of refraction n.sub.1 for a X direction polarization whose electric field component is parallel with the optical axis is n.sub.1 =1.48. And the index of refraction n.sub.2 for a Y direction polarization whose electric field component is orthogonal to the optical axis is n.sub.2 =1.52.
Accordingly, by adjusting the thickness T of the phase shifting element 99, a linearly polarized light E.sub.1 which X direction polarization phase and Y direction polarization phase are equal, as shown in FIG. 39(a) and FIG. 39(b), may be converted when passing through the phase shifting element 99 into a circularly or elliptically polarized light E.sub.2 which Y direction polarization phase lags behind the X direction polarization phase by, for example, about 90.degree., as shown in FIG. 40(a) and FIG. 40(b). A specimen of a linearly polarized light is shown in FIG. 41(a) and a specimen of circularly or elliptically polarized light is shown in FIG. 41(b).
On the other hand, in the conventional art, reflection on the surface of an optical member is prevented by forming an antireflection coating on the surface of the optical member. FIG. 42 shows an example of antireflection coating.
On the surface of an optical member 100, there is applied by deposition an antireflection coating 101 made of a dielectric material of an index of refraction slightly smaller than the index of refraction of the optical member 100, for example of MgF (index of refraction n=1.36). The antireflection coating 101 should be made of a material with an index of refraction n=.sqroot.n.sub.o, where n.sub.o is the index of refraction of the optical member 100, and should have a thickness T' such that mT'=.alpha./4 (m=0; 1, 2 . . . ) where .alpha. is the wavelength of the light the antireflection coating 101 is designed for.
However, as the phase shifting element 99 is composed by a crystalline body, it has the disadvantage that its cost is generally expensive and that the direction of its crystal axis needs to be taken in account during its manufacturing process. In addition, as the adjustment of the thickness T is performed by polishing, polishing and measuring need to be performed repeatedly, causing the process to be time consuming. Moreover, the only method of incorporating the phase shifting element 99 into another optical part is to form the phase shifting element 99 separately and to affix it thereafter. The fabrication as well as the operation of affixing the phase shifting element 99 are thus extremely complex.
Moreover, the material selected for forming the conventional antireflection coating 101 needs to be a dielectric material which index of refraction n is equal to the square root of the index of refraction n.sub.o of the optical member 100. Consequently, difficulties arise when such a dielectric material does not exist, as the antireflection coating 101 has to be formed by multiple layers using a plurality of dielectric materials.